1. Field of the Invention
The present invention relates to a route searching device, and more specifically to a dynamic route guidance system (DRGS) for searching a route to a destination based on traffic information obtained from outside facilities.
2. Description of the Related Art
Heretofore, there has been proposed a dynamic route guidance system (DRGS) which searches a route to reach a destination in the shortest period of time based on traffic information, such as a link travelling time, obtained from an information center using a vehicle information communication system (VICS), and then informs a driver of the route. In such a system, traffic information of all the links should ideally be obtained. However, in actual situations, there are usually many links where a beacon cannot be installed or where traffic information can not be obtained due to nonconformity of the beacon installed. Therefore, it is very important to deal with such links in the search of the optimum route.
Taking into account the real word situation described above, in Japanese Patent Laid-Open Publication No. Hei 7-129893, the applicant of the present invention disclosed an art for finding a route by estimating a travelling time of a link for which the travelling time cannot be obtained, based on the travelling time of surrounding links. More specifically, average car speed is found from the travelling time and link length of links which are in front of and behind the link concerned. Then, the travelling time of the link is estimated on the assumption that the link can travel at the average car speed, on the assumption that an average travelling time of links which are parallel to the link on right and left will be the travelling time of the link. Thus, accuracy in the search of the optimum route can be improved in comparison with the case that there is no traffic information whatsoever.
However, the aforementioned art is only effective when at least one link whose traffic information can be obtained is adjacent to a link whose traffic information can not be obtained and it will be difficult to accurately estimate the traffic information of a link whose traffic information cannot be obtained if the link is far from a link or links whose traffic information can be obtained. Therefore, the aforementioned art has a certain limit ineffectively utilizing the dynamic route searching system.